Studies in this laboratory on the infection of Bacillus subtilis with phate SP82 and by other investigators on the related phage SPOl have established that the subunit structure of RNA polymerase is altered sequentially following infection. We have isolated and characterized two altered enzymes: both lack the sigma subunit and contain phage-coded peptides which endow the enzyme with new transcriptional specificities. The biochemical basis for the specificity of these enzymes will be investigated by studying: 1) the role of the phage-coded peptides in the selection of transcription sites (formation of DNA-polymerase complexes), 2) the interaction of phage-coded peptide with core assembly and the specificity of transcription of the reconstituted enzymes (translation using a B. subtilis system), 3) the mechanism of sequential modification of polymerase (relative affinities of phage-coded peptide to core assembly) and 4) the synthesis of different temporal classes of early RNA by the unmodified polymerase (transcription of terminally located restriction fragments).